1. Field of the Invention
The present invention relates to a process which has better production efficiency for obtaining undrawn filaments based on polyethylene terephthalate (PET).
It also relates to PET-based undrawn modified yarns suitable for texturing by false twist drawing.
2. Description of the Prior Art
The undrawn polyester yarns which can generally be employed for the drawing-texturing operation using false twist must exhibit low crystallinity and orientation properties, so as to orient the molecules better and then to crystallize and thus set the orientation during the drawing-texturing process without degrading or breaking the filaments during the heat-setting of the yarn.
For example, it is known according to French Patent 2,151,896 that undrawn and preoriented polyester yarns (PET-POY) which can be directly employed for texturing using false twist can be obtained directly by spinning when the spinning rates and the cooling conditions are chosen appropriately. Filaments which have a desired orientation, elongation at break and crystallinity are thus obtained. The recommended spinning velocities are preferably between 2750 and 3200 m/min, but lower than 4000 m/min to prevent the strand breakages which arise during the spinning. It is generally accepted that at 4000 m/min a beginning of crystalline orientation is produced, limiting the production of the PET POY yarns to this velocity.
This is why tests have been carried out to improve the production efficiency during the spinning of PET POY yarns by introducing into the molten PET (melt) various polymers in the form of immiscible particles: for example European Patent EP 47,464 envisages the introduction of 0.2 to 10% of polyacrylate or polymethacrylate of molecular weight .gtoreq.1000 and EP 80,274 envisages the introduction of polyamide or polyethylene forming microfibrils in the filaments obtained. However, the addition of polymer in the form of fine particles presents disadvantages when applied on an industrial scale; in particular, it demands a highly sophisticated technology for obtaining mixtures which have sufficient fineness and stability with time to permit a reliable spinning without strand breakages. A technique of this kind cannot, in fact, be employed industrially.
It is also known to improve the production efficiency of undrawn polyester yarns by introducing into the polymer chain reactive sites originating from tri- or tetravalent compounds.
For example, French Patent 2,355,930 envisages the introduction of 1-15 meq of chain branching reactive sites/1 g of polymer by means of compounds such as pentaerythritol, trimesic acid, trimethylolpropane, pyromellitic acid or their esters.
EP 0,263,603 also proposes to prepare polyesters containing 2-6 meq (per g of PET) of trimesic or trimellitic acid or their esters, to obtain preoriented yarns suitable for texturing.
The use of such compounds modifies the rheology of the polymer by increasing its viscoelasticity so that the spinning of such copolymers becomes very tricky and involves major risks of strand breakages. Moreover, it is known according to EP 140,559 to prepare highly oriented and drawn polyester-based yarns containing particulate silicas which have a mean particle size smaller than 1 micron and which, after spinning and solidifying, are subjected to a conditioning in a gaseous atmosphere maintained at a temperature between 90.degree. and 200.degree. C. so as to produce their crystallization. The filaments obtained thus exhibit an improved uniformity.